Allyl alcohol is a valuable intermediate for making allyl ester derivatives, allyl monomers, 1,4-butanediol, and polymers such as styrene-allyl alcohol copolymers. Allyl alcohol can be made by isomerizing propylene oxide, but it can also be made by acetoxylation of propylene, followed by hydrolysis of the resulting allyl acetate.
Acetoxylation to produce allyl acetate is performed by reacting propylene, acetic acid, and oxygen in the vapor phase in the presence of a noble metal catalyst, typically palladium. A heated mixture of the reactants is typically contacted with a bed of supported metal catalyst, and products are separated by distillation.
Acetoxylation of propylene is well known, and many references teach ways to use various promoters to improve catalyst lifetime, productivity, or other important outcomes. See, for example, U.S. Pat. Nos. 4,647,690 and 4,571,431, which teach to make allyl acetate by reacting propylene, acetic acid, and oxygen in the presence of palladium, potassium, and bismuth in the presence of an additional rubidium or magnesium promoter. For a few additional examples, see U.S. Pat. Nos. 3,925,452, 3,917,676, 5,011,980, and 7,265,243.
A variety of schemes have been devised for purifying allyl acetate produced via acetoxylation. Usually, allyl acetate is isolated as an overhead stream in a complicated series of steps involving multiple distillations. For example, Matsuoka et al. (Daicel, Japanese Kokai Publ. No. H2-96548) describes a process in which purified allyl acetate is recovered from an acetoxylation reaction mixture using a series of five distillation columns (see FIG. 1), each of which adds considerable expense to the process. According to the reference, the last two columns (labeled 10-10 and 11-11 in the figure) are unnecessary if the allyl acetate product will be converted directly into allyl alcohol. However, that still leaves three distillation columns. In two of the three columns (labeled 5-5 and 8-8), all of the allyl acetate is recovered overhead, which is energy-intensive. Thus, there is a need to recover purified allyl acetate while avoiding the large capital and energy expense of multi-tower distillation schemes that recover all of the allyl acetate overhead.
Ideally, allyl acetate could be used to produce allyl alcohol without the need to recover the allyl acetate as an overhead distillation product. Complicating matters, however, are high-boiling impurities that form during acetoxylation. One such impurity is allyl diacetate, also known as allylidene diacetate or 1,1-diacetoxy-2-propene. It is essentially an acetal derived from the reaction of acrolein and two equivalents of acetic acid. Most references that discuss allyl acetate manufacture by acetoxylation are silent regarding the formation or removal of allyl diacetate. However, ways to remove allyl diacetate is have been discussed (see, e.g., Japanese Publ. Nos. 01-250338 (Daicel), H2-96548 (Daicel, discussed above), 61-238745 (Kuraray), and 53-071009 (Kuraray)). Usually, an allyl diacetate-containing mixture is isolated as a sidedraw from a distillation tower and simply heated to convert it to a mixture of acrolein, acetic acid, and unconverted allyl diacetate.
We discussed issues with some of these earlier processes in our own recent patent application (see copending application Ser. No. 12/322,650, filed Feb. 5, 2009), which describes a process for purifying acetoxylation mixtures that contain allyl diacetate. By contacting an acetoxylation mixture in the vapor phase with a solid acidic catalyst, we effectively decomposed the allyl diacetate to acrolein and removed it from the intermediate stream. This allowed removal of allyl diacetate, normally a “heavy” impurity, as the more volatile reaction product, acrolein. One way to practice the process involves acetoxylation, allyl diacetate decomposition, distillation to remove and recycle unreacted propylene, and distillation to remove acrolein as a light impurity.
Other impurities further complicate schemes that contemplate recovery of purified allyl acetate while avoiding its recovery overhead. As will be discussed further below, we discovered that acrolein—generated from allyl diacetate decomposition or otherwise present—reacts with acetic acid under conditions normally present in the propylene recovery column reboiler to give 3-acetoxypropionaldehyde, a heavy impurity that has not previously been recognized yet is difficult to separate from allyl acetate.
Unfortunately, the heavy impurities (allyl diacetate, 3-acetoxypropionaldehyde) cannot simply be ignored. If they are allowed to pass through to the allyl acetate hydrolysis step, they can react with the ion-exchange resin used to catalyze the hydrolysis, thereby regenerating acrolein. This acrolein can poison the resin, ultimately forcing a reactor shutdown for bed removal and regeneration.
In sum, a better way to purify allyl acetate that is produced by propylene acetoxylation is needed. A preferred process would avoid the capital and yield-loss costs of using multiple distillation columns and would enable recovery of purified allyl acetate as a bottoms product while eliminating high-boiling impurities. An ideal process could be practiced commercially in conjunction with the two-step manufacture of allyl alcohol from propylene via acetoxylation and allyl acetate hydrolysis.